1. Field of the Invention
The present invention relates to a method and circuit arrangement for simulating the machine flux of a rotating-field machine from the terminal voltages and conductor currents of at least two phases of the stator of the rotating-field machine and more particularly, to a method and circuit wherein each component of the machine flux is formed by integrating the stator terminal voltage of the same phase which is reduced by the ohmic voltage drop in that phase of the stator winding of the rotating-field machine, and the integrated inductive voltage drop is subtracted from the integral so obtained. The term "rotating-field machine" includes synchronous and asynchronous machines which can be operated as motors or generators.
2. Description of the Prior Art
A method of the kind above mentioned is known from U.S. Pat. No. 4,335,343 corresponding to German Auslegeschrift No. 2833593. The disclosure of U.S. Pat. No. 4,335,343 is hereby incorporated by reference. For regulating rotating-field machines, the actual value of the amplitude and phase of the magnetic machine flux is frequently required since the torque generated by the rotating-field machine, which is equal to the product of the machine flux and the current component perpendicular to the flux, can thereby be determined. The direct determination of the machine flux, for instance, by magnetic measuring pickups or by a measuring winding is not practical because such measurement pickups or windings are not provided in commercially available machines. According to U.S. Pat. No. 4,335,343, mentioned above, the machine flux is therefore determined by means of a computing circuit from the stator terminal voltages and conductor currents of two of the three phases of the rotating-field machine. This computing circuit is called the voltage model of the rotating-field machine. The determination of a system for simulating the machine flux begins with the fact that the voltage u.sub.i induced in each machine leg is proportional to the change of the flux and to the number of turns w in that leg: EQU u.sub.i =w(d.phi./dt)
The flux can then be calculated by integrating the voltage induced in that leg of the rotating-field machine. The terminal voltage of the machine includes, however, besides the induced voltage u.sub.i, also the ohmic and inductive voltage drops due to the copper resistance and the leakage inductance of the stator. Corrections to account for these voltage drops are made by simulating them electronically and subtracting them from the terminal voltage. Such computing circuits are realized at present exclusively by analog means. In conjunction with digital computers, however, a digital realization of the computing circuit would be advantageous because signal exchange is simplified by digital representations and the calculation of the flux can be performed by the digital computer itself, thereby saving separate analog components.
So far, digital realizations of the voltage model have failed due to the difficulties in the analog to digital (A/D) conversion of the terminal voltages. The excessively large errors that occur during sampling and A/D conversion of the terminal voltages at low machine speeds prevent simple conversion techniques. This is caused on the one hand by the fact that at low machine speeds, the terminal voltages become very small, so that the steps of the A/D converter lead to large sampling errors. Furthermore, the terminal voltages contain steep commutation peaks which are very large compared to the basic component of the terminal voltages, especially at low speeds. These commutations peaks, however, occur at random, and may or may not be present during any particular sampling instant of the A/D converter. In the subsequent integration, too large an amount is integrated if the sampling instant coincided with a commutation peak, or too low an amount if the commutation peak was not present. The integrators contained in the computing circuit integrate every error in the A/D conversion, so that incorrect values are produced very rapidly. It is therefore an object of the present invention to develop a method for simulating the machine flux of a rotating-field machine in such a manner that it can be carried out by digital means without sampling error.